1. Field of the Invention
This invention relates to a method for producing a Sixe2x80x94SiC member for heat treatment of semiconductor, for example, silicone monocrystal wafer and, more particularly, to a method for producing a Sixe2x80x94SiC member for heat treatment of semiconductor capable of reducing the contamination of the semiconductor as much as possible.
2. Description of Prior Arts
Conventionally, a Sixe2x80x94SiC material consisting of silicone (Si) and silicon carbide (SiC) has been used for a member for heat treatment of semiconductor, for example, a wafer boat for heat treatment of semiconductor (hereinafter referred to as wafer boat) because of its excellent compactness, purity and strength.
The recent higher integration of semiconductor devices, however, more strictly requests a higher purity for a heat-treating jig such as the wafer boat for heat treatment of semiconductor wafer, and the higher purity is requested also for the Sixe2x80x94SiC material forming the base material of this wafer boat.
Such conventional Sixe2x80x94SiC materials could not comply with the request of the higher purity with a content of Fe of 0.2 ppm or more and a total content of Ni, Cu, Na, Ca, Cr and K of 0.2 ppm or more as the metal impurity content even in case of a one called high-purity base material.
Further, in both the oxidation diffusion step of high temperature and the LP-CVD step of relatively low temperature, the diffusion of impurities from the wafer boat material to the semiconductor wafer is unavoidable.
Thus, giving attention to that a CVD-SiC film has excellent characteristics such as (1) excellent heat resistance and corrosion resistance, (2) an extremely small content of metal impurities, (3) suppressibility of diffusion of impurities such as the base material internal metals to the semiconductor wafer, and (4) excellent grinding characteristic with high compactness free from internal bubbles and high hardness, it has been taken as the measure for preventing the contamination to a semiconductor wafer W1, as shown in FIG. 10, to form a CVD-SiC film 21 on the surface 24 of the base material 23 of a wafer boat 22 to suppress the diffusion of the metal impurities contained in the base material 23.
However, the Sixe2x80x94SiC base material 23 used for the conventional wafer boat 22 contains, as the metal impurity concentration, 0.2 ppm or more of Fe and 0.2 ppm or more of the total content of other metallic impurities as described above. When the base material 22 contains lots of impurities in this way, the impurities are diffusively present also on the surface 21s of the SiC film 21 in the formation of the CVD-SiC film, and the semiconductor wafer W1 is consequently contaminated when placed on this wafer boat and heat-treated. This diffusion of impurities is supposedly caused by that the impurities present in the Sixe2x80x94SiC base material segregate at the tip of the CVD crystal during the growth thereof and move in the growing direction, although a general SiC film is higher in purity than the base material 22 with a bulk concentration of about 0.04 ppm for Fe.
Accordingly, in order to remove the segregated metal impurities, a strict washing with fluoric acid was performed in the past.
In order to solve such troubles, Japanese Patent Application Laid-Open No. 6-206718 discloses a high-temperature semiconductor processing apparatus formed, in stead of forming the CVD-SiC film on the Sixe2x80x94SiC base material, by use of an integrated self-standing CVD-SiC of ultra-high purity having a total metal impurity content of about 5 ppm by weight or less without using this base material.
This high-temperature semiconductor processing apparatus, however, has problems of low mechanical strength and limitation in the form of the apparatus to be manufactured because it has no base material.
Further, in cases where a nucleus for reaction is generated in the film forming process, and a crystal growth then occurs on the basis of this nucleus to generate the CVD-SiC film, projections may be formed on the surface of the CVD-SiC film although the size or number is varied depending on the synthetic condition. In order to provide the self-standing CVD-SiC, in this case, no grinding is generally performed so as to provide a strength as high as possible.
Accordingly, when a semiconductor wafer with a large diameter, for example, 8 inches or more is heated at a high temperature of 1100xc2x0 C. or higher by use of the wafer boat, the problem of such projections causing a dislocation (so-called slip) in the semiconductor wafer arises.
Therefore, a method for producing a member for heat treatment of semiconductor suitable for the heat treatment of a semiconductor wafer with a large diameter, free from contamination of the semiconductor wafer, and causing no slip has been desired.
One object of this invention is to provide a method for producing a Sixe2x80x94SiC member for heat treatment of semiconductor, which is suitable particularly for heat treatment of a semiconductor war with a large diameter and free from contamination of the semiconductor wafer. Further, another object of this invention is to provide a method for producing a Sixe2x80x94SiC member for heat treatment of semiconductor, which is free from contamination of a semiconductor wafer and causing no slip.
The method according to this invention comprises the first step of kneading a SiC powder having a total metal impurity quantity of 0.2 ppm or less with a molding assistant; the second step of molding a compact from the kneaded raw material; the third step of calcining the compact; the fourth step of purifying the calcined body; and the fifth step of impregnating the purified body with silicon within a sealed vessel provided in a heating furnace body.
It preferably further comprises the sixth step of machining the part to make contact with a semiconductor to be heat-treated into a surface roughness Ra (JIS B0601-1982) of 0.2 xcexcm or less.
In this invention, the first to fifth steps or the first to sixth steps are more preferably performed in this numerical order.
More preferably, the sealed vessel is formed of a porous carbon material having a porosity of 7-20%. The heating furnace body is provided with a mechanism for introducing and discharging an inert gas. The purifying step is performed by a heat treatment at a temperature of 1900-2000xc2x0 C. in halogen-containing atmosphere. The machining process is performed by use of a diamond blade. A CVD-SiC film forming step is performed after the machining step. A wet acid washing is performed after the CVD-SiC film forming step. A heat treatment is performed in high-temperature oxidative atmosphere after the wet acid washing step to form a silicon oxide film on the surface, and the silicon oxide film is thereafter removed by wet acid washing.
This invention is further described from another point of view.
As the purity of the SiC powder, the total metal impurity quantity is 0.2 ppm or less. When a quantity exceeding 0.2 ppm is present in the raw material stage, the ultra-high purification particularly to the inner part of the Sixe2x80x94SiC member is difficult even if the purifying treatment after kneading or each treatment step in a contamination-preventing environment is performed.
As the sealed vessel, a vessel having no through-pore at least in the thickness direction of the material constituting it is preferably used. Particularly, a vessel having a lid structure for taking in and out the purified body of the SiC member (in other words, having a fitting part) is preferred.
The reason of using the porous carbon material is as follows. When the purified body of the SiC member and the impregnating silicon are arranged within the sealed vessel, and a heating impregnation is performed at 1460xc2x0 C. or higher, the impurities left, even if slight, in the purified body are evaporated therefrom. In order to prevent the vapor from being stayed within the sealed vessel, the porous material is preferred. Further, as the material having a uniform pore distribution over the whole vessel to allow the evaporation in any parts and facilitate the higher purification and minimized in generation of particles, the carbon material is preferable.
The porosity is preferably set to 7-20%. With less than 7%, the evaporation cannot be effectively performed, and it is difficult to increase the purity of the Sixe2x80x94SiC member. Since the impurities adhesively left in the vessel are accumulated when the impregnating step is performed a plurality of times by use of the same sealed vessel for the industrial production, the possibility of contaminating the Sixe2x80x94SiC member with such impurities is enhanced. When the porosity exceeds 20%, it is difficult to effectively shield the impurities generated from the constituting material of the heating furnace body. The more preferable porosity is 10-15%.
A mechanism for introducing and discharging an inert gas is preferably provided. By the structure of carrying the inert gas within the heating furnace body, the vapor leaving the sealed vessel can be more efficiently discharged without being left in the heating furnace body.
This invention is further illustrated from another point of view.
This invention relates to an improvement in a Sixe2x80x94SiC member for heat treatment of semiconductor based on Sixe2x80x94SiC comprising SiC impregnated with Si, and the preferable form of the member is as follows.
Namely, in this Sixe2x80x94SiC member for heat treatment of semiconductor, the content of Fe is 0.05 ppm or less, as the metal impurity content of the Sixe2x80x94SiC base material, the total content of Ni, Cu, Na, Ca, Cr and K is 0.1 ppm or less, and the part at least making contact with a semiconductor has a surface roughness (Ra) of 0.20 xcexcm or less.
More preferably, this Sixe2x80x94SiC member for heat treatment of conductor has a CVD-SiC film formed on the surface of the Sixe2x80x94SiC base material, wherein the content of Fe and Na in at least the area of 10 xcexcm from the surface of the CVD-SiC film is 300 ppb or less.
A more preferable method for producing this Sixe2x80x94SiC member for heat treatment of semiconductor is as follows.
Namely, the method comprises the kneading step of kneading a SiC powder containing metal impurities of 0.05 ppm or less of Fe and 0.1 ppm or less of the total of Ni, Cu, Na, Ca, Cr, and K with a molding assistant; the molding step of molding the kneaded raw material into a compact; the calcining step of calcining the compact; the purifying step of purifying the calcined body; the impregnating step of impregnating the purified body with Si; and the machining step of machining the part to make contact with a semiconductor wafer to be put thereon of the member impregnated with silicon into a surface roughness (Ra) of 0.20 xcexcm or less.
The machining step is preferably performed by use of a diamond blade.
In the impregnating step, the calcined body of the member is housed in a sealed vessel provided within the heating furnace body to impregnate the calcined body of the member with Si.
Further, a CVD-SiC film forming step is preferably performed after the machining step.
In the CVD-SiC film formed in the CVD-SiC film forming step, the content of Fe and Na in at least the area of 10 xcexcm from the surface of this film is preferably set to 300 ppb or less.